In the drilling of subterranean bores or wells, and in subsequent downhole operations, such as required in the oil and gas industry, it is typically the case that a single string of tubing, such as a drill string, will contain multiple tools. For example, a drill string may include a drill bit, a downhole motor, an agitator, MWD tools, an under-reamer, a bypass tool and so on. The same is true of other strings as may be employed in fishing or milling operations, or strings utilised in running completions, packers, valves, plugs and the like. Activation of these tools is possible by a number of means. However, one of the most convenient and reliable methods of activating a downhole tool is to utilise fluid pressure. In such a tool, hydraulic pressure in the string, generated by surface pumps, acts across at least a part of the tool to create an activating force. Such a force may be generated by providing a flow restriction in the tool. This restriction may be a permanent feature of the tool, however such a permanent restriction will induce pressure losses in the fluid being pumped through the restriction, and will limit access to the portion of the string below the restriction. Accordingly, many tools employ activating devices, which may be in the form of balls or darts, which are dropped or pumped from surface to land on a seat provided in the tool. The activating device may wholly or partly occlude the string bore, facilitating creation of a pressure differential across the device and the seat, and thus the creation of a potentially significant activating force.
Following activation of the tool, the activating device or seat may be reconfigured to allow the device to pass through the seat, and land in a catcher provided below the tool, allowing fluid flow through the tool, and the string, to be restored.
Where multiple tools are provided in a string and it is desired to activate the tools using activating devices dropped or pumped from surface, this may be achieved by providing the tools with seats of progressively smaller diameters. For example, a first tool closer to the distal end of the string may have ball seat which is smaller than a ball seat provided in a second tool provided closer to surface. Thus, an activating ball for the first tool will be sized to pass through the ball seat in the second tool before landing on the ball seat of the first tool. However, in such an arrangement it is only possible to activate the tools in order of their proximity to the distal end of the tool, as once the second tool has been activated there is no access to allow activation of the first tool. This places severe restrictions on the utility of such tools.